Chemotactic bacteria sense chemical gradients with high sensitivity over a wide concentration range. The goal of the proposed research is to elucidate the functional architecture of the chemoreceptor signaling complex of Escherichia coli. The complex is composed of transmembrane receptor proteins that operate in cooperative teams with a histidine kinase, CheA, and a coupling protein, CheW, to amplify chemical stimuli into cytoplasmic signals that control cell motility. To determine how CheW and CheA interact with the receptor signaling domains, I propose to isolate receptor mutants that are specifically defective in those interactions, using a series of genetic and biochemical tests to identify the desired mutants. The locations of CheA and CheW in receptor signaling teams will be investigated by using single-chain receptors - molecules with two covalently connected subunits - to control the subunit composition and geometry in higher-order receptor complexes. These studies should provide general knowledge of the molecular signaling strategies of chemoreceptors and possibly valuable new insights into virulence mechanisms, as well, because chemotaxis and chemoreceptors are associated with pathogenesis in many types of bacteria.